1. Field of the Invention
The invention lies in the field of optical systems having one or more laser signal sources. More in particular, it relates to an optical system for tuning and maintaining the tuning, hereinafter referred to as stabilising, of one or more laser signal sources to a desired wave length by means of an injected optical signal. It relates further to an optical signal transmission system having a passive optical network (PON) provided with such a system for stabilising laser signal sources in network terminals of the network.
2. Prior Art
Developments are moving ever more in a direction in which local connection networks for public telecommunication services, such as telephony, are implemented as passive optical networks. Such networks form therein optical line connections between a main station (telecommunication switch) and a large number (for example 2000 or more) of network terminals for users of said services. The signal communication over such networks takes place upstream per group of connected users often at a same optical wavelength, a further differentiation to the individual user being obtained by an additional channel division, such as time division (TDM), frequency division (FDM), etc. Such networks are known, for example, from References [1] and [2] (for bibliographical details relating to the references, see hereinafter under C). Further, such a network, in which the upstream signal communication takes place on wavelength division basis (WDM), each user or group of users working at a wavelength which is specific for a user or a group of users, is known from Reference [3]. Such networks are preferably dimensioned for as large a number of network terminals as possible, an effective transmission capacity per user being offered which is as high as possible at the lowest possible costs per user. To this end the total number of network terminals is partitioned, i.e. divided into a number of groups or partitions, each group being allocated one of the number of wavelengths available for WDM. This means that each user within a group must have transmission equipment at his/her disposal which is suitable for transmitting at the wavelength allocated to the group. Problems in that respect are that transmission equipment with tuneable lasers is expensive, and that the application of lasers which must be selected for a fixed wavelength range which is specific for a group is not only expensive but also renders the network less flexible.
Another problem is that, in order to retain still detectable signals for a reliable reception in a passive optical network having a high degree of splitting out, signal amplifiers must be included at suitable points in the network. In order to limit the noise of the signal amplifiers in such a network, optical filters are also necessary. Overall, it holds true that the sensitivity of the network increases as the bandwidth of such filters is able to be chosen more narrow, and that as a result the number of users can be greater. For the upstream signal communication over such public networks, the connected users must have at their disposal transmission equipment having an optical signal capable of modulation, which is preferably generated by a relatively simple and cheap signal source. A signal source qualifying for this is, for example, a non-tuneable Fabry-Perot laser (FP laser) which operates at a wavelength around 1300 nm. Such an FP laser has the property that it possesses a number of different laser modes, each having its own wavelength within a certain wavelength band, in which it can operate. If the laser starts operating, however, the laser mode it will assume cannot be determined beforehand. The width of the wavelength band, which lies in the order of 20-55 nm, is relatively wide. Such FP lasers can therefore not be used merely in combination with narrow-band amplifiers or filters having a width of, for example, 20 to 30 nm. Furthermore, it is customary that all network terminals connected to the tree-shaped branched network of a PON, or to a tree-shaped branched part thereof, transmit at a same wavelength, at least within certain ranges of accuracy. This means that if non-selected, non-tuneable semiconductor lasers are used as signal source, a possibility must be present for tuning all these laser signal sources to the desired same transmission wavelength and keeping them tuned, i.e. stabilising them.
Optical systems are known for stabilising the wavelength of a laser signal source, such as, for example, from the References [4] and [5], which are based on a technique referred to by the term "injection locking". According to this technique, a semiconductor laser can be forced to operate in a laser mode at a desired frequency (wavelength). In this case, an external optical signal, originating from a laser signal source stabilised for a specific laser mode, called master laser, is injected into the laser cavity of a freely oscillating laser signal source, called a slave laser. If the frequency of the injected optical signal lies sufficiently close to a characteristic frequency of the slave laser, the latter will stabilise itself at the frequency of the injected signal and with a fixed relative phase with respect to the phase of the injected signal. The technique of injection locking, however, requires that the characteristic frequency of a slave laser which is to be stabilised lies within a very narrow frequency band, the locking bandwidth, around the frequency of the injected signal. This technique is in general therefore hardly or not at all suitable for stabilising two or more slave lasers simultaneously, let alone for stabilising the cheap lasers mentioned above, which are manufactured with wide tolerances. A master laser further requires optimal and well-stabilised operating conditions, such as environmental temperature control. Above a certain number of slave lasers, the optical signal originating from the master laser will furthermore need to be amplified, which is accompanied by an additional noise signal that in turn can adversely effect the stabilisation of a slave laser, as described, for example, in the Reference [5] already mentioned above.
It can be concluded, therefore, that there is a need for a technique for stabilising one or more laser signal sources at laser wavelengths within a limited wavelength band, which upon application in passive optical networks permits the use of cheap non-tuneable laser signal sources and of narrow-band amplifiers, and which, in a simple manner, allows partitioning, that is to say, wavelength allocation and stabilisation per group of network terminals, to be performed from the network.